Semiconductor materials (for example silicon, gallium arsenide, etc.) are at present conventionally implanted with dopants by devices which utilize high voltages to accelerate ions of the impurities into the surface of the semiconductors. The amount of penetration of the dopants is determined by the degree of voltage acceleration of the dopant ions, and is, for example, 0.2 microns. The annealing which necessarily follows the ion implantation has, historically, been--and still is--effected primarily by means of thermal fusion furnaces. Each of these is a long quartz tube having a diameter of, for example, four inches to seven inches, and a length of, for example, four to six feet. Heating coils are wrapped around the tube, and furnace boats are passed therethrough, each boat containing, for example 30-40 wafers. The temperature in the furnace is brought slowly up to a desired level, for example, 1000.degree. C., following which there is a holding period, following which there is a slow period of cooling. The amount of time required for a semiconductor wafer to be annealed in such a furnace is, typically, 30-60 minutes.
There have been major pressures tending toward rapid annealing (short time annealing) of large-diameter semiconductor wafers. Many papers and many patents have been written on the subject of rapid annealing, and various approaches have been made. In rapid annealing, it is typical to effect heating of the wafer to a high temperature in a short period of time, and then to hold the wafer at the elevated temperature for about one to twenty seconds. By keeping the process as short as possible, the implanted ions do not have time to diffuse into the bulk semiconductor material, and circuit speed is maximized. Referring to FIG. 9, the amount of diffusion resulting from certain prior-art rapid annealing processes is illustrated by the intermediate curve, which is seen to be very close to the "as implanted" curve.
It is extremely difficult to achieve effective, commercially-satisfactory rapid annealing of large-diameter semiconductor wafers. Major reasons for the difficulties reside in the characteristics of the wafers themselves. Some of these characteristics will now be mentioned.
The wafers may be four, five or six (or more) inches in diameter, yet are typically only 0.5 millimeters thick. This extreme thinness, in comparison to diameter, means that radiant energy transmitted to one region of the wafer will not be thermally conducted, rapidly, to another region thereof. And, as stated below, the heat--instead of being thermally conducted through the wafer to another region--will be predominently radiated away from the wafer.
Because of the wafer's size, and because the average specific heat of silicon is 1.0 joules per gram, the energy required to heat a silicon wafer to 1000.degree.-1200.degree. C. in a few seconds is substantial. For the typical 0.5 mm thick wafer, it requires 145 joules per centimeter squared in order to bring the temperature to 1200.degree. C. At a temperature of 1200.degree. C., the water will radiate (lose) 18 watts/cm squared (based on a emissivity of 0.7) over the entire area of the wafer. Thus, as an example, a four inch wafer will radiate a total of over 2.8 kilowatts when it is at 1200.degree. C. In order to hold the wafer at 1200.degree. C., it is necessary for the wafer to continuously absorb 36 watts/centimeter squared for one-sided heating, or 18 watts/cm.sup.2 for double-sided heating.
Referring next to the optical properties of the semiconductor materials, it is emphasized that most have a very high index of refraction (3.0 to 4.0) in the wavelength range of 0.3 to 4.0 microns, which means that the materials reflect from 30 to 40 percent of the incident radiation. This is many times higher than what would be the case relative to, for example, glass. Not only is there much reflection, but there is a large amount of transmission of the radiation through the wafers when they are relatively cold. From 40-50 percent of the incident radiation in the range from 1.1 to 8 microns is transmitted through the wafer at temperatures below 500.degree.-600.degree. C. Thus, the wafers are radiating, reflecting and transmitting large amounts of energy.
A further characteristic of the wafers is that they are highly subject to thermal and physical stresses, being easily distorted instead of remaining flat as desired. Furthermore, regions thereof may tend to ripple when thermally shocked.
An additional important characteristic is that relatively long "rapid annealing" tends to reduce adverse effects caused by disuniform heating, that is to say, transmission of radiant energy in differing amounts to different regions of the wafer. However, such relatively long "rapid annealing" is not desired, because it slows production and tends to increase downward diffusion of the dopant and thus reduce circuit speed.
The problems of rapid annealing, and prior-art attempted solutions of such problems, are well summarized by two articles, one of which is: "Rapid Wafer Heating: Status 1983" by Pieter S. Burggraaf (Semiconductor International, December, 1983, pp. 69-74). A somewhat less recent overview is "Short Time Annealing" by T. O. Sedgwick (Journal of the Electrochemical Society: Solid-State Science and Technology, February, 1983, pp. 484-493). Both of such articles are hereby incorporated by reference herein.
The Burggraaf article emphasizes the great need for uniformity, stating (p. 70) that" . . . wafer-temperature uniformity is perhaps the most important issue that each vendor has addressed in designing its specific system. Wafer-temperature uniformity is important in rapid wafer heating to minimize slip (crystal dislocation) and wafer flatness distortions that occur at high temperature. Wafer-temperature uniformity also affects dopant-activation and junction-depth uniformities. Uniform heating, in fact, is a major challenge in making rapid wafer heating a production tool . . . Wafer-temperature uniformity requires that the radiation field be very uniform."
Relative to the mention of junction-depth uniformities in the quoted statement, it is emphasized that since the wafers are cut up into many hundreds of elements, and it is important that all of these elements be alike, variations in junction-depth resulting from temperature disuniformity are one of the factors which have been adverse to bringing rapid annealing into a viable production-line status.
In the above-cited Sedgwick article, it is pointed out that there is need to operate at as high a temperature as possible in order to both activate the implanted ions and relieve several types of point defects. Applicant is of the opinion that much of the high temperature work has been adequate in regard to temperatures, but has involved scanning laser beams which heat in small localized areas and develop strains, slippage, ripples and other damage.
A further major factor relating to the viability of rapid annealing is wafer contamination, which is discussed in (for example) the Burggraaf article (pp. 70 and 71). To prevent contamination, it is important to rapidly heat the wafer to 800.degree.-1100.degree. C. (or higher) without touching it or contaminating it in any way. Thus, for example, use of a preheated plate at high temperatures is distinctly undesirable, for reasons including the fact that material from the plate would enter the wafers in the indicated temperature range.
Other very important factors relating to the question of whether or not rapid annealing apparatus achieves widespread production-line use are the cost of the apparatus and the cost and difficulty of operating and mantaining it. Efficiency, simplicity, relative compactness, ruggedness, ease of maintenance, etc., are of major importance here as in other production-line operations. And, of course, speed of operation--as well as versatility and accuracy (for example, accurate temperature control)--are paramount considerations.
Among the myriad attempts to achieve uniformity of temperature in rapid annealing and other processes, there frequently occur two approaches. One is to transmit the radiant energy through diffusers, for example, quartz sheets or housings that have been sand-blasted or otherwise treated so as to diffuse light. The other approach, which is often used with the first, is to employ susceptors which engage the wafers and aid in heat distribution. Both of these approaches are not desired, and the need therefor is eliminated by the present method and apparatus. One reason the approaches are not desired is that they drastically increase the time required to heat and cool the wafers, thus increasing cycle time and wasting enormous amounts of power.